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Stock-Based Compensation and Employee Benefit Plan - Equity Incentive Plan (Additional Information) (Details) - $ / shares
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares available for grant (in shares) 14,762,327 1,037,616 2,768,450  
Weighted average exercise price, outstanding options (in dollars per share) $ 28.11 $ 26.42 $ 23.85  
2012 Equity Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Minimum grant percentage of fair value of common stock       85.00%
Minimum grant percentage of fair value of shareholders       110.00%
Minimum grant percentage of fair value of voting stock       10.00%
Weighted average exercise price, outstanding options (in dollars per share) $ 30.95      
2012 Equity Incentive Plan | Class B common stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of outstanding options (in shares) 11,600,000      
2018 Equity Incentive Plan | Class A common stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance (in shares) 13,300,000      
Number of common stock reserved for issuance (in shares) 26,700,000      
Stock option | 2012 Equity Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares available for grant (in shares) 14,800,000      
Restricted Stock | 2012 Equity Incentive Plan | Class B common stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance (in shares) 3,107,101